The present invention relates generally to a tractor cab which is vibration isolated from the tractor chassis and more specifically to an adjustable mount system for eliminating low frequency vibrations introduced into the cab by tire lugs.
Tractor cabs have been provided on modern farm tractors to reduce noise, vibration and objectionable atmospheric conditions. Use of resilient mount assemblies is common to isolate the cab and controls from vibrations in the tractor chassis. Such mount assemblies are described, for example, in U.S. Reissue Pat. No. Re. 29,123 which shows use of forward and rearward pairs of resilient mount assemblies connecting the cab and control console to the chassis. A vibration problem often still exists in cabs with this and similar type mountings when utilized with tractors having lugged tires. As the tires rotate, forces resulting from the tire lugs contacting the ground are transmitted to the tractor chassis and ultimately to the cab through the resilient mounts. Although the mounts usually reduce the forces transmitted to the cab, at certain low frequencies corresponding to the natural frequencies of the cab, which depend upon the spring rate of the mount system and the mass of the cab, the entire cab vibrates. At certain ground speeds extremely annoying vibrations can occur. Often the annoying vibrations are present at a commonly used field-working speed. Such vibrations are very noticeable and objectionable to the operator who can see relative motion at low frequencies between the cab and other parts of the tractor. In addition, as cabs and isolation systems become more advanced and other noises and vibrations are reduced, the farmer increasingly notices the vibration caused by the tire lugs.
Heretofore, mount systems have provided no simple or effective means of eliminating the annoying vibration in the cab caused by the tire lugs. Attempts to decrease the vibration at resonance using different resilient material in the mounts or by adding shims between the chassis and the cab have generally decreased isolation effectiveness at higher frequencies. As a result, the higher frequency chassis vibratory modes and chassis noise were more readily sensed by the operator. Utilizing increased mount damping decreases the motion of the cab at the resonant frequency but also increases the noise level at frequencies above resonance.